[U-Boot] [PATCH 0/3] dm: add dev_get_reg() for getting device node's reg

[Stephen Warren]

On 01/05/2016 08:38 AM, Przemyslaw Marczak wrote:
> Hello,
>
> On 01/04/2016 09:06 PM, Stephen Warren wrote:
>> On 12/29/2015 01:47 AM, Przemyslaw Marczak wrote:
>>> Hello Stephen,
>>>
>>> On 12/16/2015 08:07 PM, Stephen Warren wrote:
>>>> On 12/16/2015 11:53 AM, Stephen Warren wrote:
>>>>> On 12/15/2015 09:32 AM, Przemyslaw Marczak wrote:
>>>>>> commit: dm: core: Enable optional use of fdt_translate_address()
>>>>>>
>>>>>> enables device's bus/child address translation method, depending
>>>>>> on bus 'ranges' property and including child 'reg' property.
>>>>>> This change makes impossible to decode the 'reg' for node with
>>>>>> '#size-cells' equal to 0.
>>>>>>
>>>>>> Such case is possible by the specification and is also used in
>>>>>> U-Boot,
>>>>>> e.g. by I2C uclass or S5P GPIO - the last one is broken at present.
>>>>>
>>>>> Can you please explain the problem you're seeing in more detail?
>>>>> Without
>>>>> any context, my initial reaction is that this is simply a bug
>>>>> somewhere.
>>>>> That bug should be fixed, rather than introducing new APIs to hide the
>>>>> problem.
>>>>
>>>> Ah, I guess the problem is caused by the following code in
>>>> __of_translate_address():
>>>>
>>>>      bus->count_cells(blob, parent, &na, &ns);
>>>>      if (!OF_CHECK_COUNTS(na, ns)) {
>>>>          printf("%s: Bad cell count for %s\n", __FUNCTION__,
>>>>
>>>
>>> Yes, and this is what my previous patch 'fixes'.
>>>
>>> [1] https://patchwork.ozlabs.org/patch/537372/
>>>
>>> However Linux makes the translate in the same way.
>>>
>>>> That's because the function assumes it's called for MMIO addresses.
>>>> However, reg values for I2C devices aren't MMIO addresses, so those
>>>> assumptions don't apply. So, there is an argument for introducing some
>>>> new functionality. I'm not sure that a whole new function is the
>>>> correct
>>>> way to go though. Rather, the existing translation functions should be
>>>> enhanced to know the location of root of the address space that
>>>> contains
>>>> the address that's being translated. Then, translation can stop there.
>>>
>>> This is okay but then, all device tree blobs should be defined in a
>>> proper way.
>>
>> Well, why shouldn't that be true? There are rules for how DTs must be
>> constructed. Nobody should expect DTs that violate those rules to work
>> in any particular way.
>>
>>> The problem is, that there are some additions and various assumptions in
>>> the drivers, e.g. the exynos gpio driver (s5p_gpio.c) is checking the
>>> reg's property value for each bank. But the driver in Linux hardcodes
>>> those values, however for both cases this is wrong, because the gpio
>>> regs could be mapped with ranges.
>>
>> It sounds like there are many bugs to fix:-)
>>
>
> Unfortunately... :(
>
>>> Even that issues above, I would prefer introduce a function or modify
>>> the existing one to allow keeping this as it is.
>>
>> Adding an extra function sounds OK, although I stand by my comment that
>> the caller should pass in a parameter indicating the root of the address
>> space, so that both #address-cells and #size-cells can be checked all
>> the way up the chain, and #size-cells should only be allowed to be 0 at
>> the root of the translation, not at any intermediate point.
>>
>>>> Something like skipping the check on ns in the above code if parent ==
>>>> addr_space_root_offset, and also terminating the for (;;) loop in that
>>>> function under a similar condition.
>>>>
>>>> This would allow for translation to occur for buses other than the
>>>> CPU's
>>>> root MMIO space, yet not attempt to translate across known address
>>>> space
>>>> boundaries (i.e. where address translation is known to be impossible).
>>>
>>> To achieve this functionality, it should be enough to take my first
>>> patch [1]. And then if no "ranges" is defined, then we have 1:1
>>> translation.
>>
>> I don't think so; that patch removes all checks on #size-cells rather
>> than only removing/ignoring the check at the root of the address space.
>>
>>> I think, that it is safe, but then we will have a different assumptions,
>>> than in the Linux - is it acceptable?
>>
>> Both Linux and U-Boot should conform to the DT specification. So, if
>> there's a difference between the two, there's likely a bug.
>>
>>
>
> According to your comments with the new parameter, I think that we don't
> need this. As Simon wrote in one of his reply:
>
>   "How would the caller know this root?".

This is a facet of the hardware.

The root of the MMIO address space is the root of the DT.

The root of any other kind of address space is the IO controller that 
"hosts" the address space, i.e. the I2C or SPI controller.

Every device knows semantically what its address represents, and hence 
can trivially determine the root node of the address space.

Device driver writers shouldn't have to care about this, so likely some 
form of helper function should be provided by I2C/SPI/... subsystems to 
hide these details. IIRC (although I haven't looked in a while) this is 
exactly how/why the Linux kernel avoids this kind of issue; the 
I2C/SPI/... subsystem, handles parsing of reg properties before any 
device-specific driver is invoked.